Wave-propagation in rocks saturated by two immiscible fluids

G Papageorgiou,M Chapman

doi:10.1093/gji/ggx128

Abstract

Accurate modelling of the dependence of seismic wave speed from frequency and fluid content is crucial to the quantitative interpretation of seismic data. Dispersive effects such as squirt flow become important at a critical frequency that is proportional to fluid mobility. When a porous medium is partially saturated it is not clear how the respective fluid mobilities are to be averaged. Building on previous work, we use a nonzero, static capillary pressure parameter and a relative permeability model to simulate the effects of squirt flow in a realistic sand saturated by water and CO2 where the CO2 can be in either the liquid or supercritical phase. We show that the effective fluid follows a mixing law similar to Brie's empirical model and the effective frequency depends both on the relative permeability model and the capillary pressure parameter which can potentially lead to a dispersive effect in the seismic band.

Highlights

1 INTRODUCTION Assessing the impact of partial saturation on seismic waves propagating in porous media is vey important for a variety of applications
This is an important observation since the standard linear solid model has been experimentally verified against partial saturation, frequency-dependent data obtained in the laboratory (Chapman et al, 2017)
KCO2 Kw and the model reduces to Gassmann’s model with the effective fluid of equation (19) at low frequencies. We account for these inter-fluid effects through the introduction of a new non-dimensional parameter and, while we offer no explicit modelling of its dependencies, we argue physically that it must lie between one and the ratio of the fluid bulk moduli

Summary

Introduction

Assessing the impact of partial saturation on seismic waves propagating in porous media is vey important for a variety of applications. Chapman capture and storage (CCS) projects relies on accurately estimating the amount and location of CO2 from seismic surveying. There is an increasing recognition that the relationship between partial fluid saturation and velocity is more complex than that predicted by the Gassmann-Wood formula, with patchy saturation models gaining in popularity (Eid et al, 2015). Fully saturated rocks are known to exhibit velocity dispersion with a characteristic frequency controlled by fluid mobility (Batzle et al, 2006). The importance of developing a consistent approach which can handle the effects known to be important for both full and partial saturation has been noted by Ghosh et al (2015)

Objectives

Results

Discussion

Conclusion